1. Field of the Invention
The invention is based on a power switching apparatus. Such a power switching apparatus is used in systems which are operated with voltages above 1 kV. The power switching apparatus is used to protect equipment in the system against both slowly rising currents (overcurrents) and quickly rising currents (short-circuit currents).
2. Discussion of Background
A power switching apparatus of the type mentioned above is described in EP 0 655 760 A. In this power switching apparatus, a switching device and a PTC thermistor are arranged, connected in series with one another, in an electrical conductor carrying a high voltage. Above a threshold value, the PTC thermistor limits the current carried in the electrical conductor. A varistor connected in parallel with the PTC thermistor prevents undesirable voltage spikes occurring at the onset of current limiting, which could load the PTC thermistor to an unacceptably high level. Changes in the physical characteristics of the PTC thermistor which indicate that the current is being limited, such as geometric dimensions, temperature or electrical resistance, are detected as a signal. The detected signal is supplied to a tripping apparatus, which opens the switching device.
A switching apparatus having a plurality of series-connected PTC thermistors is specified in WO 90/00825 A1. In this apparatus, 10 of 30 PTC thermistors are in each case connected in series, and the three resultant series circuits are connected in parallel with one another. This switching apparatus allowed an alternaing current of only 2 A to be limited at a voltage of 6 kV. A series resistance of about 500 .OMEGA. was required for this purpose.
A resistor having a PTC characteristic and which is distinguished by a high rated-current capacity and a high withstand voltage is already known from EP 0 548 606 B1. This resistor has a plurality of series-connected PTC thermistors. A varistor, which dissipates locally occurring overvoltages, is connected in parallel with each of the PTC thermistors. The PTC thermistor and the varistor have a common contact surface. This results in the temperature distribution in the thermistor being homogenized, thus effectively countering the risk of local overheating. At the same time, this increases the rated-current capacity.